High-index contrast distributed bragg reflector

ABSTRACT

A distributed Bragg reflector has a sectioned waveguide with a high index of refraction. The waveguide is disposed within a medium having a relatively low index of refraction. Each of the sections of the waveguide are coupled with a thin waveguide having a high index of refraction. In one embodiment, the wire and waveguide sections are formed of the same high index material.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C.119(e) to U.S. Provisional Patent Application Serial No. 60/350,294,filed Jan. 17, 2002, No. 60/375,959, filed Apr. 25, 2002, No.60/382,686, filed May 22, 2002, and No. 60/382,676, filed May 22, 2002,all of which are incorporated herein by reference.

[0002] This application is also related to co-pending U.S. patentapplication (1153.073US1) entitled “High-Index Contrast WaveguideCoupler,” and filed on the same date herewith, all of which isincorporated herein by reference.

FIELD OF THE INVENTION

[0003] The present invention relates to optical components, and inparticular to a high index-contrast distributed Bragg reflector.

BACKGROUND OF THE INVENTION

[0004] Distributed Bragg reflectors (DBRs) are frequently used simply asreflectors. Other applications include vertical-cavity surface-emittinglasers (VCSELs), filtering and wavelength division multiplexing (WDM).The performance of DBRs (e.g., bandwidth and reflectivity) improves withthe index contrast of the materials used to form the DBR. In otherwords, one material has a high index of refraction, and the othermaterial has a low index. However, losses also increase as a consequenceof light diffraction in the low-index regions. Conventional solutionsfor reducing losses are usually either reduction of index contrast, oran excessive shortening of the low-index region, both leading to adecrease in device performance. In order to provide high reflectivity insmall sizes, diffraction losses must be reduced to a minimum.

SUMMARY OF THE INVENTION

[0005] A distributed Bragg reflector has a sectioned waveguide with ahigh index of refraction. The waveguide is disposed within a mediumhaving a relatively low index of refraction. Each sections of thewaveguide are coupled with a thin waveguide having a high index ofrefraction. In one embodiment, the wire and waveguide sections areformed of the same high index material.

[0006] In one embodiment, the high index thin waveguide has a muchsmaller diameter or width than the waveguide. The thin waveguide issurrounded by the low index material and is coupled to adjacent segmentsof the waveguide near the center of the segments. For a four periodstructure, three waveguide segments are coupled to each other and theremainder of the waveguide by four thin waveguides.

[0007] In one embodiment, the waveguide is formed from Si having anindex of refraction of approximately 3.48, and the medium having a lowindex of refraction is air having an index of refraction of 1. With oneset of geometries, the effective indices of refraction are calculated at3.27 and 1.45 in the high and low index regions respectively. Lossesvary with the width of the thin waveguide, as does the reflectivity.Optimizing the width of the thin waveguide provides high reflectivityand low losses for a distributed Bragg reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a perspective view of a thin waveguide coupled segmentedvertical waveguide

[0009]FIG. 2 is a graph showing losses and reflectivity as a width ofthe thin waveguide of FIG. 1 is varied.

[0010]FIG. 3 is a perspective view of one example of a planar opticalwaveguide having thin waveguide coupled waveguide segments.

[0011]FIG. 4 is a perspective view of a further example of a planaroptical waveguide having thin waveguide coupled waveguide segments.

[0012]FIG. 5 is a block diagram of a waveguide coupler.

[0013]FIG. 6 is a perspective view of yet a further example of a planaroptical waveguide having thin waveguides coupling waveguide segments.

DETAILED DESCRIPTION OF THE INVENTION

[0014] In the following description and the drawings illustrate specificembodiments of the invention sufficiently to enable those skilled in theart to practice it. Other embodiments may incorporate structural,logical, process, and other changes. Examples merely typify possiblevariations. Individual components and functions are optional unlessexplicitly required, and the sequence of operations may vary. Portionsand features of some embodiments may be included in or substituted forthose of others. The scope of the invention encompasses the full ambitof the claims and all available equivalents. The following descriptionis, therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

[0015] A distributed Bragg reflector is shown generally at 110 inFIG. 1. A waveguide having a high index of refraction has a first end115, and several sections of approximately equal diameter 120, 125, and130 positioned between the first end 115 of the waveguide and a secondend of the waveguide 140. Section 120 is coupled to the first end 115 bya thin waveguide section 145. Section 120 and 125 are coupled by a thinwaveguide 150. Section 125 and section 130 are coupled by a thinwaveguide 150, and section 130 and second end 140 are coupled by a thinwaveguide 160. Each thin waveguide has a high index of refraction. Thewaveguide structure, including the ends of the waveguide, the sections,and thin waveguides are surrounded by medium 170 having a low index ofrefraction.

[0016] In one embodiment, the medium is air, having a index ofrefraction of 1. The waveguide structure is formed of silicon, and hasan index of refraction of approximately 3.48. With one set ofgeometries, the effective indices of refraction are calculated at 3.27and 1.45 in the high and low index regions respectively. Losses varywith the width of the thin waveguide, as does the reflectivity.Optimizing the width of the thin waveguide provides high reflectivityand low losses for a distributed Bragg reflector.

[0017]FIG. 2 shows a graph 210 of reflectivity 220 and losses 230 for an8 period structure while a width of the thin waveguide structures isvaried at 210. The length of each thin waveguide is approximately 100nm, and the width of the thin waveguide is varied between zero and 500nm, the width of the waveguide. The length is varied to satisfy thecondition 2(N_(h,eff)l_(H)+N_(L,eff) l_(L)) is equal to a wavelength,λ₀, of 1.55 um, where N_(h,eff)l is the effective index of refraction ofthe high index material, l_(H) is the length of each section of thewaveguide, N_(L,eff) is the effective index of refraction of the lowindex material, and _(L) is the length of each thin waveguide. For thinwaveguide widths, W_(L), of between zero and approximately 100 nm, thereflectivity is near 100%. It drops off as the thin waveguide widthincreases and becomes approximately zero as the width approaches thewidth of the waveguide, 500 nm. The structure has losses of about 1.6%when the thin waveguide width is zero. As the thin waveguide width isincreased to approximately 50 nm, the losses are practically eliminated(below 0.04%) at 240. At this width, the reflectivity is still near100%. As the width increases, the losses also increase until about 200nm, and then the losses decrease again.

[0018] As illustrated by the graph in FIG. 2, an optimal width or rangeof widths exists, where the reflectivity is near 100%, and the lossesare minimal. This range can easily be found for a number of differentstructures, employing varying widths and lengths of waveguides and thinwaveguide structures beyond the example provided. Device geometry mayalso vary in either two-dimensional or three-dimensional structures,while keeping the thin waveguide structure consistently significantlysmaller in width or cross section respectively than that of the sectionsof the waveguide.

[0019] Wide bandwidth is strongly desirable in some embodiments, and isintrinsically associated with high-index contrast between high and lowindex sections. The same high index contrast also allows the device toachieve very desirable characteristics of high-reflectivity with only afew periods, which in turn facilitates the fabrication of very smallreflectors and other devices.

[0020] As an example, a one-dimensional analogy of a two-dimensional orthree-dimensional distributed Bragg reflector can be used for giving anidea of the dependency of reflectivity on index contrast. In thisanalogy, it is considered that alternate high and low-index sectionshave the effective index of the respective fundamental modes. Thisanalogy leads to a good approximation of the real 2D or 3D structure,provided that losses are not very high. Considering that the electric oroptical path length is odd multiples of λ₀/4 for each section, thefollowing expression is obtained for the reflectivity (R) of thereflector:${R = {1 - \frac{2}{1 + \left( \frac{n_{H}}{n_{L}} \right)^{2p}}}},$

[0021] where p is the number of periods, and n_(H) and n_(L) are theeffective indices of the high and lo-index sections respectively. Inthis analogy, it is seen that the higher the index contrast, the closerthe reflectivity is to the unity.

[0022] The geometry of the structure is designed in one embodiment foroperating at a single spatial mode of propagation, although multimodeoperation is also within the scope of the invention. Geometricdimensions can be varied accordingly in order to achieve the desiredspectral and loss performances for different frequencies or wavelengthsof the electromagnetic spectrum. The geometry is also related to theBragg-order of the distributed Bragg reflector through the definition ofthe electric of optical path length.

[0023] Any Bragg-order can be implemented using the proposed structures,leading to substantial reduction of losses. In fact, the higher theBragg-order, the easier it is to fabricate the device as a directconsequence of the enlargement of some dimensions, but losses becomeeven more significant by means of constructive coupling to radiationmodes along specific directions. This effect opens up plenty of room foruse of the proposed structure, by means of the fact that it acts just asa suppressor of diffraction losses, which are direction associated tocoupling the radiation modes along the reflector.

[0024] The reflector structure is implemented not exactly periodic inone embodiment in order to adjust the spectral response as desirable.The extremity of the proposed distributed Bragg reflector structures maypresent any property and geometry, similar to or different from anyother section inside the reflector, in order to promote the desirableeffect when coupling to an external device or medium.

[0025] In one embodiment, a distributed Bragg reflector is fabricatedusing any dielectric material or combination of dielectric materials inorder to form successive sections of high and low refractive indices.Materials such as silicon, GaAs, AlAs, InP and other III-V compounds areused in one embodiment for the high-refractive index sections. Othermaterials may also be used. Materials such as air, SiO₂, AlO₂, andoxides of III-V compounds may be used for the low-refractive indexsections, as well as for surrounding media. Other materials, includingorganic materials may also be used in further embodiments.

[0026] In FIGS. 3 and 4, planar optical waveguides forming distributedBragg reflectors are shown at 310 and 410. Reflector 310 compriseswaveguide sections 315, 320, 330, 340 and 350 coupled by thin waveguides355, 360, 370 and 380. In this case, the wires are substantiallycentered on each waveguide section, and have much smaller widths andheights than the waveguide sections. They are essentially floatingbetween the sections at the geometric center of the cross section ofsuch sections.

[0027] In FIG. 4, reflector 410 comprises waveguide sections 415, 420,425, 430, and 435 coupled by thin waveguides 440, 450, 460 and 470. Inboth the waveguides, the indices of refraction for the waveguidesections and wires are high compared to medium surrounding them. Thethin waveguides in this embodiment are the same height as the waveguidesections, but are much narrower. They are easily formed using a singlephotolithograph step to define both the waveguide sections and the thinwaveguides.

[0028] The planar optical waveguides are formed on a substrate or bufferof a material with lower refractive index. The waveguides are buried ina material with lower refractive index or left with air as the upperlayer. Fabrication is performed using lithographic processes (optical ore-beam) for patterning the device onto the substrate, such as silicon oninsulator substrates, or another appropriate bulk or buffered substrate.Following patterning, reactive ion etching or appropriate depositionprocesses, depending on the type of substrate utilized, are performed tocomplete the device. The devices may be performed in many differentmanners, as the resulting structure and difference in index ofrefraction in surrounding medium are easily obtainable by many differentprocesses, including those yet to be developed.

[0029] The vertical structure of FIG. 1 is formed by epitaxial growth ordeposition using any process, such as MBE, CVD, MOCVD, evaporation andsputtering, as well as any other available process. Materials used areusually III-V compounds and alloys, as well as oxides thereof, but othermaterials may also be used. Conventional processes used for fabricatingreflectors in vertical cavity surface emitting lasers are inherentlyappropriate for fabrication of the vertical structures. In a veryconventional process, MBE is used for epitaxially growing or alternateGaAs and AlAs layers, (optical or e-beam) lithography is used forpatterning the cross sectional regions, and plasma or reactive ionetching is employed for transferring the pattern. Using this example,selective etching of AlAs with respect to GaAs layers is performed as afinal step in order to obtain the corresponding narrow AlAs thinwaveguide layers alternated by wide high-index GaAs layers. This leadsto a structure geometrically similar to that of FIG. 1, which presents agenerally cylindrical cross section, although other cross section shapesmay also be used.

[0030]FIG. 5 shows the use of the invention to serve as a couplerbetween two optical fibers or between an optical fiber and anotheroptical device. The coupler would have an optical fiber 510 in themiddle (or equivalent waveguide) and a wire-lens 515 and 520 extendingfrom each end (same material for both thin waveguide and fiber). The twothin waveguides could be different to allow coupling from a device withone mode to a device with another. Another version of the coupler couldbe an optical fiber (or equivalent device) with a thin waveguide (madeof a material with a different refractive index) embedded in it. Theends of the thin waveguide could be either flush with the ends of thefiber or could protrude beyond it. The tapered structure between anoptical tip and waveguide may also be utilized as wire-lenses 515 and520.

[0031] In a further embodiment, an optical type fiber having embeddedone or more thin waveguide structures made with a different index ofrefraction material than the surrounding fiber. The thin waveguidestructures could be located towards the middle of the optical-type fiberor around the periphery. In one embodiment, each thin waveguide tocarries a separate optical signal. The thin waveguides are spaced fromeach other such that signals carried on the separate thin waveguidestructures would not interfere with each other nor would there be asignificant loss of signal from the thin waveguide. The application ofthis is for telecommunications and perhaps in any optical system wheresimultaneous transmission of multiple separate optical signals aredesired—such as an optical equivalent of multiwire cables used incomputers now (for use in an optical computer). One method of makingsuch fibers comprises arranging high and low index fibers and heatingand drawing them a thin fiber of substantially parallel fibers.

[0032]FIG. 6 is a perspective view of a further waveguide formed inaccordance with the present invention. A waveguide having a high indexof refraction has a first end 615, and several sections of approximatelyequal cross section 620, 625, and 630 positioned laterally from thefirst end 615 of the waveguide. Section 620 is coupled to the first end615 by a thin waveguide section 645. Section 120 and 125 are alsocoupled by a thin waveguide, and section 125 and section 130 are coupledby a thin waveguide, which are not visible in this view, but aresubstantially the same as thin waveguide section 645.

[0033] Each thin waveguide has a high index of refraction. The areabetween the waveguide sections is surrounded by cladding comprisingmedium 670 having a low index of refraction.

[0034] The waveguide sections are formed on a silicon on insulator (SIO)structure and are sandwiched between two SiO₂ layers 675 and 680. In oneembodiment, the medium 670 is air, and in further embodiments, SiO2 isused. While the size of each of these structures may be varied dependingon desired properties, the one set of dimensions is provided as just oneexample. Each thin waveguide section is approximately 150 nm in length,and the thicker sections 620, 625 and 630 are approximately 200 nm inlength. The first end of the waveguide has a cross section ofapproximately 500×500 nm. The SiO₂ layers 675 and 680 are approximately150 nm thick, and the thin waveguide sections such as 645 areapproximately 150 nm thick.

CONCLUSION

[0035] High-index contrast distributed Bragg reflectors are used ashigh-reflectivity and wide-bandwidth reflectors. A high-Q resonantcavity may be formed by placing high-reflectivity mirrors at itsextremities. High-index contrast reflectors enable small dimensions forhigh-reflectivity reflectors, allowing small and high-Q resonantcavities to be formed. Vertical-cavity surface-emitting lasers may beformed due to the ability to form small and high-Q resonant cavities inthe vertical direction. Arrays of such lasers may be used in opticalswitching and other applications. Such an array requires small cavities,which are facilitated by the present invention. Standalone or seriesassociation of high-index contrast distributed Bragg reflectors can beused to provide filters presenting sharp and steep spectral responses.Both reflective and transmissive properties may be utilized in suchfilters. Yet further uses for the high-index contrast distributed Braggreflectors enable wavelength division multiplexing by providing theability to tailor special filters presenting sharp and steep spectralresponse allowing selective transmission and reflection of desiredspectral ranges.

1. A distributed Bragg reflector comprising: a sectioned waveguidehaving a high index of refraction; areas having a low index ofrefraction disposed between the sections of the waveguide; and wire-lenssections having a high index of refraction coupled between the sectionsof the waveguide.
 2. The distributed Bragg reflector of claim 1, whereinthe index of refraction of the waveguide sections is approximately equalto the index of refraction of the wire-lens sections.
 3. The distributedBragg reflector of claim 1 wherein the wire-lens sections have a crosssection much smaller than the cross section of the waveguide sections.4. The distributed Bragg reflector of claim 1 wherein the effectiveindex of refraction of the waveguide sections is approximately 3.27 andthe effective index of refraction of the areas between the waveguidesections is approximately 1.45.
 5. The distributed Bragg reflector ofclaim 1 wherein the waveguide sections are either two dimensional orthree dimensional.
 6. The distributed Bragg reflector of claim 1 whereinthe waveguide sections are formed from a dielectric material selectedfrom 111-V compounds.
 7. The distributed Bragg reflector of claim 6wherein the waveguide sections are formed from a dielectric materialselected from the group consisting of GaAs, AlAs and InP.
 8. Thedistributed Bragg reflector of claim 1 wherein the areas have a lowindex of refraction are formed from materials selected from the groupconsisting of air, SiO₂ and AlO₂.
 9. The distributed Bragg reflector ofclaim 1 wherein the areas having a low index of refraction are formedfrom materials selected from the group consisting of oxides of III-Vcompounds.
 10. The distributed Bragg reflector of claim 1 wherein thesectioned waveguide is disposed in a substrate in a planar or verticalmanner.
 11. The distributed Bragg reflector of claim 1 and furthercomprising mirrors placed at each end of the sectioned waveguide. 12.The distributed Bragg reflector of claim 11 and further comprisingmultiple such reflectors arranged in an array.
 13. The distributed Braggreflector of claim 1 wherein sizes of the elements are modified toachieve desired spectral response.
 14. A reflector comprising: aplurality of waveguide segments spaced along an axis; a plurality ofwire lens segments disposed co-axially between the waveguide segments;and a medium disposed between the waveguide segments having a lowerindex of refraction than the index of refraction of the waveguide andwire lens segments.
 15. The reflector of claim 14 wherein the mediumextends to encompass waveguide segments.
 16. The reflector of claim 14wherein the wire lens segments have a cross section of the same shape asthe cross section of the waveguide segments.
 17. The reflector of claim16 wherein the cross sections are rectangular or generally circular. 18.The reflector of claim 14 wherein the wire lens segments have a heightapproximately equal to a height of the waveguide segments, but anarrower width.
 19. The reflector claim 14 wherein the wire lenssegments are disposed approximately at the geometric center of thewaveguide segments.
 20. The reflector of claim 14 wherein areflectivity, R, is approximately equal to:${R = {1 - \frac{2}{1 + \left( \frac{n_{H}}{n_{L}} \right)^{2p}}}},$

where p is a number of periods, and n_(H) and n_(L) are the effectiveindex of refraction of the waveguide and wire lens segments, and theindex of refraction of the medium.
 21. The reflector of claim 14,wherein the wire lens segments have a length varied to satisfy thecondition 2(N_(h,eff)l_(H)+N_(L,eff)l_(L)) is equal to a desiredwavelength, λ₀, where N_(h,eff)l is the effective index of refraction ofthe segments, l_(H) is the length of each waveguide segment, N_(L,eff)is the effective index of refraction of the medium, and l_(L) is thelength of the wire lens segments.
 22. An optical device comprising: anoptical waveguide having two ends; a pair of reflectors coupled to eachend of the optical waveguide, each reflector comprising: a plurality ofwaveguide segments spaced along an axis; a plurality of wire lenssegments disposed co-axially between the waveguide segments; and amedium disposed between the waveguide segments having a lower index ofrefraction than the index of refraction of the waveguide and wire lenssegments.
 23. A distributed Bragg reflector comprising: a waveguidehaving coaxial, spaced thick sections with a high index of refraction;areas having a low index of refraction disposed between the thicksections of the waveguide; thin waveguide sections having a high indexof refraction coupled between the sections of the waveguide; and a layerof insulation disposed along a side of the thick and thin waveguidesections parallel to the axis.
 24. The reflector of claim 23 and furthercomprising a second layer of insulation disposed along an opposite sideof the thick and thin waveguide sections.
 25. The reflector of claim 24wherein the layers of insulation comprise SiO₂.
 26. The reflector ofclaim 25 wherein the areas having a low index of refraction compriseSiO₂.
 27. The reflector of claim 23 wherein the thin lens segments havea height approximately equal to a height of the waveguide segments.